Referring to FIG. 1, we show, in schematic representation, a cross-sectional view of a write head for a magnetic disk system. The magnetic field needed to perform the write operation is generated by flat coil 16 made up of a number of turns (typically between about 8 and 8). Surrounding the flat coil are upper and lower pole pieces 12 and 11 respectively, made of a magnetic material such as nickel-iron. These pole pieces are joined at one end (on the left in this figure) and are separated by region 14 at the other end. The magnetic field that is generated by flat coil 16 ends up being concentrated in region 14. It is sufficiently powerful that the fringing field that extends outwards away from 14 is capable of magnetizing the magnetic storage medium over whose surface 15 the head `flies`. The distance between region 14 and surface 15 is typically between about 10 and 50 nm. In practice, lower pole 11 is also used as a magnetic shield for the reading assembly that is located immediately below it. The latter can comprise many layers and is not shown in the figure. For this reason, pole 11 is usually referred to as the shared pole.
In FIG. 2 we show a more detailed view of the parts that make up region 14, the gap structure. Rather than being a simple gap between poles 11 and 12, the gap structure is made up of two additional magnetic sub-components, sub-shared pole layer 21 and sub-top pole layer 22, separated by gap layer 24 of a non-magnetic material. The area of these sub-poles is significantly less than that of the opposing flat portions of 11 and 12 that make up the gap region 14 in FIG. 1. Thus, the introduction of sub-poles serves to concentrate the magnetic flux across gap 24 making for more intense fringing fields in its vicinity.
In U.S. Pat. No. 5,285,340 in February 1994, Ju et al. describe a pole tip structure, similar to that shown in FIG. 2, which can be formed by a single photolithographic process. Their basic approach was to use a photoresist mold inside which the layers 21, 22, and 24 could be grown by electroplating, following which the photoresist is removed in a conventional manner. While the process of Ju et al. represents a significant improvement to the art, there are associated problems. In particular, as the inside dimensions of the photoresist mold grow smaller, there is a growing tendency for the plated layers that are laid down inside it to acquire curved, non-planar surfaces. This, of course, reduces both the strength as well as the spatial resolution of the gap. The present invention provides an explanation for this problem as well as a solution for it.
A routine search of the prior art was conducted but no references that teach the solution disclosed in the present invention were encountered. Several references of interest were, however, found. For example, in U.S. Pat. No. 5,940,253, Mallary et al. show how photoresist may be used to prevent edge shorting in a laminated plated pole structure.
In U.S. Pat. No. 5,652,687, Chen et al. describe a modified sub-top pole in the shape of a U thus changing the aspect ratio of the gap. A method, based on using a very thin photoresist layer, for manufacturing this structure is disclosed in U.S. Pat. No. 5,802,700 (Chen et al.). In U.S. Pat. No. 5,812,350, Chen et al. modify the behavior of the gap by including a layer of nickel iron having a different composition from the standard Ni.sub.81 Fe.sub.19.